1. Field of the Invention
The instant invention relates generally to variable displacement axial piston type fluid energy translating devices and more specifically to the control devices therefore.
2. Description of the Prior Art
A common type of axial piston fluid energy translating device is a pump or motor which includes a housing having a rotatably mounted barrel with a plurality of circumferentially spaced cylinder bores. A port plate is interposed between the barrel and the inlet and working ports of the device to alternately connect each cylinder with the inlet and working ports of the device as the barrel is rotated. Within each bore is a piston which is connected by shoes to a thrust plate assembly mounted on a pivotal rocker cam assembly which reciprocates the pistons to pump fluid as the barrel is rotated.
In one form of variable displacement axial piston pump, the rocker cam assembly is pivoted about an axis perpendicular to the axis of rotation of the barrel to vary the inclination of the thrust plate assembly. This changes the stroke of the pistons and consequently changes the displacement of the pump. In such pumps, a control device is provided to vary the inclination of the rocker cam.
In the United States Letters Patent 3,803,987 to Knapp, and assigned to the assignee of the instant invention, a variable displacement axial piston pump is shown with a displacement control device which operates a pair of control pistons to pivot a rocker cam in a cam cradle. The control pistons receive pressure fluid through a servo valve which includes a valve spool and follower valve sleeve. A manually operated hydraulic actuator supplies pressure fluid to move the spool relative to the sleeve to supply fluid to one control piston or the other. The sleeve is connected through a mechanical feed-back linkage to the rocker cam. As the cam is moved the sleeve is displaced until it reaches a neutral position with the spool which cuts off fluid flow to the control pistons.
In United States Letters Patent 3,739,691 to Bobier, a variable displacement axial piston pump is shown with a rocker cam assembly mounted on a pivotable yoke. As the yoke pivots, the rocker cam assembly is pivoted with respect to the cylinder barrel to change the stroke of the pistons. An L-shaped arm on the yoke has a slot which engages a connecting pin. This pin is connected to a displacement control device. In one embodiment, the displacement control device is a piston mounted in a housing bore and positioned by a thumb screw.
Such prior art displacement control devices are connected to the rocker cam by a mechanical linkage. A disadvantage of such devices is the tolerances inherent in mechanical linkages which may cause free play and may make precise positioning of the rocker cam difficult.
Additionally, such prior art displacement control devices may lack functions such as automatically centering the pump, or by-passing working fluid and applying or releasing a brake for a vehicle operated by the pump. These and other functions are desirable in certain applications as described hereinafter.